The present invention relates to a spray gun which utilizes compressed air for atomizing and spraying materials such as paint, and more particularly to a spray gun which can sufficiently atomize the material when the pressure of compressed air is 1 kgf/cm.sup.2 or less and which can avoid the splashing of paint particles and prevent environmental pollution.
A spray gun, generally called an air spray gun, is used in the painting and coating processes of spraying paint materials. Air spray guns which utilize compressed air for atomization are generally categorized as either internal mixing types or external mixing types.
The internal mixing type spray gun is one by which the material paint and the compressed air are mixed together within an atomizing head of the spray gun, and an air-liquid mixture is injected and atomized from the atomizing nozzle. In the external mixing type spray gun, a paint dispensing hole and an air dispensing hole open outwardly from the atomizing head, and the liquid paint flow dispensed through the paint dispensing hole is dispersed and atomized by the air flow, which is diffused and blown around the paint flow.
These spray guns utilize the suction force of the compressed air for the spraying of the paint or the paint is sprayed by a force-feed unit such as a pump at a pressure of several kgf/cm.sup.2, and atomization is not achievable by merely spraying the paint material. The air is used as a force for atomizing the paint, and a pressure of 2-5 kgf/cm.sup.2 is usually applied.
For example, in a conventional internal mixing type spray gun 100, as shown in FIG. 11, a paint nozzle 101 opens into an air cap 102. The nozzle hole 104 is opened and closed by a needle valve 103 to control the flow of the paint. Compressed air is supplied from around the nozzle hole 104 in order to disperse the paint within the air cap 102, and the paint is sprayed out from the spray hole 105, which is located in opposing relation to the nozzle hole 104 and opens into the center of the air cap 102.
Such conventional internal mixing type spray gun is generally used as a special spray gun to spray wall paint or adhesive in cases where strict smoothness of the painted surface is not required, because the size of the sprayed particles is coarser than that produced by external mixing type guns.
In contrast, external mixing type guns are generally used as so-called spray guns and are known as being suitable for various types of paint materials and various painting conditions. The common features of this type spray gun are that the nozzle hole of the paint nozzle is located at the center, and the nozzle hole faces to the outside of the atomizing head. An annular air hole is provided around the nozzle hole, and compressed air is blown at a pressure of 3-5 kgf/cm.sup.2 as it surrounds the paint flow from the nozzle hole. Namely, the paint and the compressed air are dispensed separately and are mixed and atomized in front of and outside the atomizing head. Usually, most of the spray guns of this type are designed in such a manner that lateral air holes are provided on both sides, and compressed air is sprayed from both sides to said spray flow in order to adjust the shape of the spray pattern. Therefore, a spray flow sprayed in a circular pattern at the center may be flattened by changing the air flow pressure and quantity from the lateral air holes. In the case of this spray gun, better atomization is achieved when compressed air quantity (or pressure) is increased, such that the painted surface is provided with a good finish due to the spraying of finer particles. However, the paint is splashed more in this case.
In conventional type spray guns as described above, there is the problem with the splashing of paint because it is atomized by air, and unfavorable results occur in terms of paint adhesion efficiency and environmental hygiene. Above all, this trend becomes more conspicuous when air spray pressure is increased, thus it is desirable to spray at lower pressure. On the other hand, it is important to have finer atomized particles for better finishing of the painted surface. For this purpose, it is necessary to atomize the paint with high pressure air, and this is contradictory to the elimination of paint splashing. At present, the spraying is performed by accepting the problem of splashing, and sacrificing paint adhesion efficiency and environmental hygiene.
However, importance will be increasingly placed in the future on the effective utilization of the material resources and maintenance of the environment, and spray guns which can atomize material at low pressures are in demand. The means to atomize the paint material is not necessarily limited to compressed air, and a method is generally practiced, in which paint material is dispensed at high pressure from the nozzle and is atomized through its collision with atmospheric air. However, spray guns for industrial purposes require high pressures of 100 kgf/cm.sup.2, and are also dangerous because a special type pump is used. For this reason, spray guns are used in which paint material pressure is decreased to several tens of kgf/cm.sup.2 and in which compressed air is simultaneously dispensed. But, the above problems remain because air pressures for spray guns are not sufficiently low, and satisfactory spray guns are not yet available.